Quantum length dependence of conductance in oligomers: First-principles calculations

Abstract

In recent experiment it was found that for a quantum length dependence of conductance of oligothiophene-CH2 molecules under low bias ͓Xu et al., Nano Lett. 5, 1491 ͑2005͔͒, the longer molecule has larger conductance. Due to the experimental motivation, we calculated the conductance of a similar organic compound, oligothiophene, by means of the first-principles method. Our calculations show a similar quantum length dependence of conductance in the low bias region and an oscillated length dependence of conductance in the high bias region. The transport behaviors are determined by the distinct electronic structures of the molecular compounds. The length dependencies of conductance for several other oligomers are calculated to show the diversity in the transport behaviors of molecular wires. The results show that Ohm's law is not valid for the molecular conductance anymore, and for the low bias region, Magoga's law is not applicable for some molecular wires, for example, oligothiophene dithiolates.

Electron transport through ribbonlike molecular wires calculated using density-functional theory and Green’s function formalism

“…2, which shows the room-temperature, zero-bias conductance of the above families of molecules, with different numbers n of fused benzene rings and lengths up to 18 Å. In contrast with a recent suggestion 29 that the low-bias conductance of para-acenes should decrease exponentially with length, Fig. 2 shows that for both the para-and meta-acene series, no such exponential dependence exists.…”

First-principles study of length dependence of conductance in alkanedithiols

“…16 assumes that the HOMO-LUMO gap remains constant to expect an exponential increase of resistance with the length, for the damping factor is regarded as a function of the gap. However, the HOMO-LUMO gaps of some oligomers do not keep constant but decrease with the increase of the length of molecules, [46][47][48] which leads the heavy overlap of the tails of HOMO and LUMO levels, the large conductance, and the unusual length dependence. These molecules do not obey the exponential law even at low bias because the transmission regime is not nonresonance.…”

Large negative differential resistance and rectifying behaviors in isolated thiophene nanowire devices

“…Furthermore, it is notable that the I-V curves of both system A and B show several prominent NDR peaks. However, these large NDR and rectifying behaviors along with the oscillation effects were not found in previous experimental and theoretical works, [15][16][17][18][19] where the 3D gold electrodes were used to connect the 2T molecule. Thus, as a check, Figure 2(b) also displays the I-V curve of the same 2T molecule sandwiched between two 3D gold electrodes as depicted in Figure 1(b).…”

“…The calculated I-V curves display the prominent negative differential resistance and rectifying behaviors along with the quantum oscillatory effects, a resonant-tunneling diode performance, which are different from the results from previous theoretical and experimental studies. 18,19 Our results can provide fundamental guidelines for designing of molecular electronic devices. The proposed 1D device could be experimentally realized primarily with scanning probe microscopy techniques, which is capable of visualizing device structure as well as molecular orbitals.…”